Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $71.38   13.423%   $80.9613   $80.96   $80.95   $81.00  
  Purchase #2 $61.42   6.839%   $65.6205   $65.62   $65.60   $65.60  
  Purchase #3 $96.58   11.463%   $107.651   $107.65   $107.65   $107.70  
  Purchase #4 $136.80   12.562%   $153.9848   $153.98   $154.00   $154.00  
  Purchase #5 $93.38   3.323%   $96.483   $96.48   $96.50   $96.50  
  Purchase #6 $207.50   11.816%   $232.0182   $232.02   $232.00   $232.00  
  Purchase #7 $11.55   3.855%   $11.9953   $12.00   $12.00   $12.00  
  7 purchase totals = $678.61       $748.7141   $748.71   $748.70   $748.80